The present invention relates to a ball joint for a prop unit and to a prop unit provided with this joint.
U.S. Pat. Nos. 3,414,302 and 3,101,961, French Patent Specifications Nos. 2,042,262, 901,628 and 1,458,003 and German Patent Specifications Nos. 2,015,368 and 506,246 all describe ball joints in which a skirt having spherical and concentric external and internal support surfaces is one of the elements of a ball joint whilst the other element is a central spherical head fastened to an outer annular element having spherical convexity. These Patent Specifications, and more particularly U.S. Pat. No. 3,414,302, show these known joints connecting parts which are not in alignment, such as those which are required in steering linkages for automobile vehicles or for obliquely transmitting a force applied by the rod of a jack.
In all these known devices the possibility of inclination is limited as the edge of the skirt bears against the spherical head. Accordingly, if such devices were used in a support unit liable to bear heavy loads, of the order of tens of tonnes, the following disadvantage would exist: when the load was an oblique thrust the edge of the skirt or of the annular element would support a considerable proportion of the load, with the risk of deformation or jamming.
This disadvantage is likewise not avoided by the device described in German Patent No. 1,122,914.
In the ball joints applied to props which are described in U.S. Pat. No. 3,481,573 and French Patent No. 1,226,776, these patents do not avoid the disadvantages of thrusts very close to the articulation centre, that is to say supporting impermissible forces when the load is oblique.
It is an object of the present invention to provide improvements making it possible for ball joints to be used with props of support units whilst avoiding thrust support pressures which are impermissible for the metal and whilst increasing the angle of articulation.
As has already been proposed in the patents mentioned above, it is possible to use joints having two degrees of angular freedom, of the cardan joint type, but joints of this kind entail difficulties of alignment when a plurality of joints are required, as is often the case with the head-pieces of support units. Furthermore, joints based on the cardan principle are very bulky.